Yahoo's resolve to regain ground lost in the lucrative Internet ad market is signaled by its deal to supplant Google's search engine on Firefox's Web browser in the U.S.

The five-year alliance announced Wednesday will end a decade-old partnership in the U.S. between Google Inc. and the Mozilla Foundation, which oversees the Firefox browser, according to The Associated Press. The tensions between Google and Mozilla had been rising since Google's introduction of the Chrome browser in 2008 began to undercut Firefox.

Google's current contract with Mozilla expires at the end of this month, opening an opportunity for Yahoo to pounce.

Even though Chrome is now more widely used, Firefox still has a loyal audience that makes more than 100 billion worldwide search requests annually.

Yahoo is hoping to impress Firefox users as the Sunnyvale, California, company sets out to prove that it's still adept at Internet search after leaning on Microsoft's technology for most of the results on Yahoo's own website for the past four years.

Google accounted for 90 percent, or about $274 million, of Mozilla's royalty revenue in 2012. Mozilla hasn't released its annual report for last year.

Besides dropping Google in the U.S., Mozilla is also shifting Firefox to Baidu's search engine in China and Yandex in Russia. Firefox users still have the option to pull down a tab to pick Google and other search engines as their preferred way for looking up information online.

Yahoo Inc. CEO Marissa Mayer, a former Google executive, hailed the Firefox agreement as Yahoo's most significant partnership since forging the Microsoft deal in 2009.

"We believe deeply in search – it's an area of investment and opportunity for us," Mayer wrote in a Wednesday blog post.

Yahoo plans to unveil a "clean and modern" search engine on Firefox next month and then roll out the new model on its own website early next year, Mayer wrote.

The redesign will primarily affect how Yahoo's search engine's results are displayed, and not the way that requests are processed. The search technology will continue to be provided by Microsoft Corp. as part of a 10-year deal Yahoo signed in 2009, according to Mel Guymon, Yahoo's vice president of search.

In various public remarks since becoming Yahoo's CEO two years ago, Mayer has expressed disappointment with Microsoft's search technology. That has spurred speculation that she might renegotiate or end the Microsoft search partnership next year when Yahoo has an option to re-evaluate the deal. Yahoo currently receives $88 of every $100 in revenue generated from ads posted alongside the search results on its website.

Those payouts have helped Yahoo boost its revenue from search advertising for 11 consecutive quarters, compared with the previous year. Despite those gains, more searches have been shifting to Microsoft's Bing search engine, causing Yahoo to slip further behind its rivals.

Yahoo is expected to end this year with a 5.6 percent share of U.S. search advertising revenue, down from 6.6 percent in 2012, according to the research firm eMarketer.